In the 4th edition Dungeons & Dragons campaign I am currently running the group spent level 1 fighting kobolds and zombies. Now at level 2 they got into combat with humans for the first time. Now in previous editions of Dungeons & Dragons you would have created a level 2 human bandit using the same rules as for creating a player character. You'd declare him to be a fighter or rogue, and give him stats and abilities accordingly. 4th edition doesn't work like that any more, there you'll find a human bandit listed in the Monster Manual. Human NPCs you are supposed to fight in 4E follow the rules for monsters, not those for players.

In a way that is inevitable, due to player characters being so much more complex in 4th edition. For a player it is nice to have lots of powers and choices when creating his character. But for a DM who just needs a couple of bandits, it would be quite a workload. So instead the NPC gets a simple monster template with only a few powers. And you can use rules for "solo" or "elite" or "minion" monsters to vary the NPCs hit points way from 1 to much more than a player character of that level could have.

The downside of that is that players will quickly realize that the NPCs don't follow the same rules as they do. The NPCs use powers that the players don't have access to, NPC wizards cast spells the player wizards can't learn, and so on. It makes the NPCs seem less human, less real. But that seems to be a common theme of 4th edition: More game, less believable world. Still beats MMORPGs, though.

The DM has to really point out how good the Human "monster" is good at what he does. Example, in my campaign there was a halfling bounty hunter NPC which used a bolas with a "monster" power to Restrain (save ends) a PC with a hit or throw sevral daggers at different targets.

The PCs have to understand that NPC have access to different powers depending on their backgrounds. It makes for a much more fun game when the NPC don't use the same powers as the PCs.

In my campaign a monster used caltrops as a minor action power, my player collected the caltrops and I allowed him to use them but as a Move action instead. It seemed a harsh decision but the player understood that for balance I couldn't let the PCs acquire new somewhat strong powers just with gear.

But if a player is willing to learn to use the special item by spending a feat or replacing an existing power (at-will, encounter or daily depending on the potential strenght of the new power) than it's ok. Example to restrain with the bolas could be an encounter power or daily if it does effects on a missed (slowed) and more damage.

Well, the added benefit of pen & paper is that if your players really, really want a skill that an enemy had, you can give it to them. Well, more likely make them undergo an arduous quest for it, but still. You are free from someone else's ideas of what balanced and fair are.

Given that D&D 4th has (roughly) 45 classes, and each of those have a hundred or so powers, my group doesn't take the idea that monsters have different powers than they do as immersion breaking.

As a side issue, I really love that D&D 4 doesn't "build" monsters the same way that D&D 3 did. The exception based Monster creation in 4 is very easy to use and makes for quick prep. And the different monster roles make interesting encounters. Overall I rate monster design as one of the real high points of this edition.

That the PC's can't have the NPC's powers makes it a less believable world?

Really the whole 'anyone can be anyone' that comes with anyone being able to pick anyone elses powers just makes a harmoginised world to me. Which I can dig because a blander world seems more 'realistic' to me, but it's still blander.

And what about beserkers in AD&D who start with two attacks, but are NPC only?

D&D has always been asymetric, with all the weird monsters.

Now in previous editions of Dungeons & Dragons you would have created a level 2 human bandit using the same rules as for creating a player character.As I recall that's not true in AD&D - PC classes are relatively rare - like one in fifty are a PC class. Even in third they had NPC classes which were weaker.

First, players expect asymmetry. Without asymmetry they would win only 50% of the time which would suck in a game with permadeath.

Next older version of D&D did work like this. My AD&D monster manual from 1979 had level 0 bandits in it that are weaker than a player would be.

Lastly I don't think it damages immersion at all. Lord of the Rings has nazgul who are clearly more powerful than regular mortals as well as cannon fodder monsters that players chew up and spit out. It doesn't damage the story. Even, arguably, real world conflict has seen some troops who are much more powerful than others. Feudalism was based on this. that a knight is of more martial worth than a couple of peasants.